(a) Field of the Invention
The present invention relates to a laser device used as a light source for optical recording and optical communications. In particular, the invention relates to a surface-emitting semiconductor laser.
(b) Description of Related Art
In recent years, semiconductor lasers have been growing in demand as light sources for optical recording and optical communications. Among them, a surface-emitting semiconductor laser has been expected as a power-saving, low-cost light emitting element due to its features such as low threshold current, round beam spot, easiness in direct connection with an optical fiber and adaptability to inspection in a wafer state.
Hereinafter, referring to FIG. 7, an explanation is given to a configuration of a known surface-emitting semiconductor laser disclosed in Japanese Unexamined Patent Publication No. 2003-188471. FIG. 7 is a sectional view illustrating a configuration of a conventional surface-emitting semiconductor laser.
As shown in FIG. 7, the conventional surface-emitting semiconductor laser includes an n-type semiconductor substrate 101 made of GaAs, on which an n-type mirror 102 made of a 40.5-fold stack of laminated layers of Al0.3Ga0.7As and Al0.9Ga0.1As, undoped spacer layers 103 made of Al0.6Ga0.4As, an active layer 104 made of a Al0.11Ga0.89As/Al0.3Ga0.7As triple quantum well sandwiched between the spacer layers, a p-type AlAs layer 105, a p-type mirror 106 made of a 30.5-fold stack of Al0.3Ga0.7As/Al0.9Ga0.1As laminated layers and a p-type GaAs layer 107 are stacked in this order. The n-type mirror 102 is doped with Si at a Nd concentration of 1×1018 cm−3 (donor concentration) and the p-type mirror 106 is doped with Zn at a Na concentration of 7×1017 cm−3 (acceptor concentration) Further, the p-type GaAs layer 107 is doped with Zn at a Na concentration of 1×1019 cm−3.
The p-type AlAs layer 105 includes oxidized regions 105A and a non-oxidized region 105B laterally sandwiched between the oxidized regions 105A, thereby narrowing current to the non-oxidized region 105B. An upper part of the n-type mirror 102 and the layers arranged thereon are shaped into a post structure 113. Further, a contact electrode 109 having an opening 112 for laser emission is formed on the p-type GaAs layer 107. An interlayer insulating film 110 made of SiNx is formed to cover the outer edges of the contact electrode 109, the side surfaces of the layers forming the post structure 113 and the exposed surface of the n-type mirror 102. Further, a wiring electrode 111 is formed on the interlayer insulating film 110. The wiring electrode 111 covers part of the interlayer insulating film 110 which is in contact with the outer edge of the contact electrode 109, thereby contacting the contact electrode 109. On the opposite surface of the semiconductor substrate 101, an n-side electrode 108 is formed.
When a bias voltage is applied between the n-side electrode 108 and the wiring electrode 111 in the thus configured surface-emitting semiconductor laser, current is narrowed to the non-oxidized region 105B of the p-type AlAs layer 105 to inject carriers into the active layer 104. As a result, light generated in the active layer 104 is oscillated in a resonator made up of the n-type mirror 102 and the p-type mirror 106 and then emitted out of the opening 112.
With the above-described conventional configuration, however, electrical conduction of holes is restricted by a spike in a valence band caused at an interface of a Al0.3Ga0.7As/Al0.9Ga0.1As heterojunction in the p-type mirror 106, which causes inconvenience that extremely high voltage application is required for operating the surface-emitting laser. Further, since the p-type mirror 106 is shaped into the post structure and small in cross sectional area, the resistance component due to the influence of the spike becomes significantly large. This leads to a problem of heat generation in the p-type mirror 106. Since the cross sectional area of the p-type mirror 106 is small, the generated heat is not sufficiently emitted out, resulting in reduction in light emitting efficiency of the laser.